Servo motor controlled transfer system for automatic press line

ABSTRACT

A transfer system for an automatic press line which includes a gripper for transferring workpieces between presses. The gripper is supported on the ends of a pair of transfer arms projecting from a corresponding pair of pantographs. The pantographs are manipulated by a pair of ball screw assemblies operated by a corresponding pair of servo motors. The servo motors are electrically connected to the press drive for synchronizing movement of the transfer assembly with the press line. A pair of cams connected to the press drive are associated with a tachometer and a resolver for generating electrical input signals for the control system associated with the servo motors.

United States Patent Cummens et a1.

1 SERVO MOTOR CONTROLLED TRANSFER SYSTEM FOR AUTOMATIC PRESS LINEInventors: Joseph R. Cummens; Francis E.

Heiberger, both of Elmhurst, Ill.

[73] Assignee: Danly Machine Corporation, Chicago, Ill.

Filed: July 8, 1971 Appl. No.: 160,799

[56] References Cited UNITED STATES PATENTS 8/1965 Danly ..318/85 X11/1962 E1161] et a] 318/578 Primary Examiner-T. E. LynchAttorney-Wolfe, Hubbard, Leydig, Voit & Osann, Ltd.

[ 5 7] ABSTRACT A transfer system for an automatic press line whichincludes a gripper for transferring workpieces between presses. Thegripper is supported on the ends of a pair of transfer arms projectingfrom a corresponding pair of pantog'raphs. The pantographsaremanipulated by a pair of ball screw assemblies operated by acorresponding pair of servo motors. The servo motors are electricallyconnected to the press drive for synchronizing movement of the transferassembly with the press line. A pair of cams connected to the pressdrive are associated with a tachometer and a resolver for generatingelectrical input signals for the control system associated with theservo motors.

5 Claims, 11 Drawing Figures W J/PV/ l/fZZ/fl/f 1? //Z na /z; u/rzm/mrwf/JfiJ/f/ 1 PATENTEBAPR 1 71975 SHEET H (]F 6 rem afz/wffwzz wad SERVOMOTOR CONTROLLED TRANSFER SYSTEM FOR AUTOMATIC PRESS LINE DESCRIPTION OFTHE INVENTION The present invention relates generally to transfermechanisms for power presses and, more particularly, to an improvedtransfer mechanism for an automatic press line, and an improved controlsystem for synchronizing the operation of the transfer mechanism withoperation of the press line.

In US. Pat. No. 3,199,439 to James C. Danly, a system is disclosed forsynchronizing the operation of a line of independently driven presses,each equipped with a variable speed drive, at any desired operatingspeed up to a limit imposed by the mechanical capabilities of thepresses. The synchronization is achieved by feeding back to each pressdrive a phase error signal obtained by continuously comparing the pressphase position with an externally generated reference phase position.With that arrangement, if any given press tends to get out of step, aphase error signal is supplied to the press drive to increase ordecrease the press speed as necessary to maintain the press in step.Thus, any corrective action required is provided when only minor changesin press speed suffice and, therefore, the necessary corrections arereadily made despite the high inertias of the presses.

Although the control system described in the aforementioned patentprovides excellent synchronization, it is relatively costly in that itrequires a complex power take-off from the main press drive.

Accordingly, it is one of the primary objects of the present inventionto provide an improved transfer mechanism and control system, for usewith a synchronized automatic press line, which simplifies the powertake-off from the main press drive system.

It is another object of the invention to provide such an improvedtransfer mechanism, and associated control system, which has minimumpower requirement.

It is a further object of the invention to provide such an improvedtransfer mechanism and control system which can be efficiently andeconomically manufactured, installed, and operated.

Yet another object of the invention is to provide such a transfermechanism and control system which is relatively light in weight, andwell balanced.

A still further object of the invention is to provide such an improvedtransfer mechanism and control system which has good working rigidityand in which the travel of the actuating component is relatively shortin comparison with the travel'of the working components. In thisconnection, a related object of the invention is to provide such amechanism which can provide any desired working stroke.

Still another object of the invention is to provide such an improvedtransfer mechanism which can be easily and quickly adjusted or modifiedto transfer different types of work pieces, and to provide accuratepositioning of the work pieces. 1

A further object of the invention is to provide such an improvedtransfer mechanism which has a compact overall assembly, therebypermitting relatively close spacing of adjacent presses, with attendantreductions in floor space requirements.

Other objects and advantages of the invention will become'apparent fromthe following detailed description taken in conjunction with theattached drawings, in which:

FIG. 1 is a side elevation, partially in section and partiallydiagrammatic, illustrating a portion of a typical press line embodyingthe present invention;

FIG. 2 is an enlarged vertical cross section of one of the gripperassemblies utilized in the press line of FIG.

FIG. 3 is an enlarged side elevation of one of the transfer mechanismassemblies utilized in the press line of FIG. 1;

FIG. 4 is an enlarged end shown in FIG. 3;

FIG. 5 is a section taken along line 5-5 in FIG. 4;

FIG. 6 is a section also taken along the line 5-5 in FIG. 4, but withthe transfer mechanism in a different operative position;

FIG. 7 is a section also taken along the line 5-5 in FIG. 4, but withthe transfer mechanism in another different operative position;

FIG. 8 is a side elevation of the same transfer mechanism shown in FIG.3, but in a different operating position;

FIG. 9 is a block diagram of an electronic control system associatedwith one of the servo motors included in the transfer mechanism assemblyshown in FIGS. 1 through 8;

FIG. 10 is a diagrammatic perspective of the driving mechanism for oneof the presses shown in FIG. 1; and

FIG. 11 is an enlarged vertical cross-section taken along line 11-11looking into the cam box in FIG. 10.

While the invention will be described in connection with one particularembodiment, it is to be understood that it is not intended to limit theinvention to any particular embodiment. To the contrary, the intentionis to cover all alternatives, modifications and equivalents fallingwithin the spirit and scope of the invention.

Turning now to the drawings, and referring first to FIG. 1 there isshown a portion of an automated press line intended for performingsuccessive operations on a workpiece W. For purposes of illustration,only three presses have been shown, indicated as press A, press B, andpress C, together with their associated transfer and conveyormechanisms, but it will be understood that the press line may inpractice be extended to include any number of similar units.

As here illustrated, the presses are identical and of generallyconventional construction. For example, the press A includes a massivebase 10 which is anchored below the floor 11 and a frame 12 topped by acrown l3. Reciprocatingly mounted in the press frame 12 is a slide 14with cooperating upper and lower dies 16 and 17, respectively. To drivethe press A, a motor 18 is mounted on the press crown 13 and coupled tothe press drive which, in turn, is coupled to the press slide 14.FoFcTa'iity, corresponding parts in the three presses have beenidentified in the drawings by identical reference numerals with theaddition of the distinguishing suffix b for elements of press B, andsuffix c for elements of press C.

The coupling between the press motor, press drive, and slide 14 is notshown in detail, since it may be entirely conventional. If the detailsof this drive system and coupling are of interest, reference may be hadto elevation of the assembly the trade brochures and descriptivebulletins of the press manufacturers, aswell as to Danly US. Pat. Nos.3,199,439 and 3,199,443. For the present purposes it suffices to simplynote that the press drive motor and its associated speed controlsprovide an electrically responsive, variable speed drive for the pressesA, B, C, and that the drive for the press A is independent of thesimilar variable speed drives provided for presses B and C and the otherpresses in the line.

As illustrated, for transporting the workpieces W down the press line,there are respective conveyors 21 leading up to and away from the inputand output sides of each press. Further, each press is equipped with aninput transfer mechanism 22 and an output transfer mechanism 23. In theillustrative transfer mechanism, the workpiece W is gripped between apair of jaws 24 and 25 (FIG. 2) in a conventional gripper assembly 26.Since the gripper assembly 26 does not form a part of the presentinvention, it need not be described in detail herein, and it willsuffice to simply note that when the pneumatic actuator 27 for themovable lower jaw is advanced, the jaw 25 is pivoted upwardly to .gripthe workpiece against the underside of the upper jaw 24, as illustratedby the broken line drawing of jaw 25 in FIG. 2. When the movement of thepneumatic actuator 27 is reversed, the movable jaw 25 is returned to itsopen position by means of a biasing spring 28. The entire gripperassembly is mounted on a gripper mounting plate 29 carried on the lowerend of a transfer arm 30 which is an integral part of the transfermechanism to be described in more detail below.

In accordance with one important aspect of the present invention, thetransfer mechanism includes a pantograph comprising four pivotallyinterconnected links, with the elongated transfer arm extending from oneof the links of the pantograph and carrying the gripper assembly on theend thereof. Thus, in the illustrative arrangement, a pair ofpantographs and 40a which are mirror images of each other areinterconnected to work together in unison. Each pantograph 40 and 40acomprises four pivotally interconnected links 41, 42, 43 and 44, and41a, 42a, 43a, and 44a, respectively. Both pantographs 40 and 40a aremounted on a common main pivot shaft 45 which extends through, and isrigidly connected to, a pair of spaced frame plates 46 and 47.

For the purpose of actuating the pantographs 40 and 400, the links 42and 42a thereof are connected .to a pair of integrally connected pins 48and 49 which can be displaced both vertically and horizontally, relativeto the main pivot shaft 45, by means of a biaxial ball screw drivearrangement mounted between the spaced frame plates 46 and 47. When thepins 48 and 49 are moved either vertically or horizontally, the entirepantograph 40 is pivoted around the fixed pivot shaft 45, and theindividual links of the pantograph are pivoted relative to each otherabout the four pivot points at the four corners of the pantograph wherethe links are connected to each other.

One of the advantages of the pantographic linkage between the biaxialdrive system and the gripper assembly 26 is that the pantographamplifies any given displacement of the pins 48 and 49 by the biaxialdrive system. Consequently, the actuating portion of the transfermechanism, i.e., the biaxial drive system connected to the pins 48 and49 between the frame plates 46 and 47, is operated at a lower velocityand acceleration than the working portion of the mechanism, i.e., thegripper assembly 26. For example, if a working stroke of 72 inches isdesired for the gripper assembly 26 mounted on the end of the transferarm 30, the pantograph 40 may be designed to provide a 4-to-1amplification, so that an actuating movement of only 18 inches isrequired at the pins 48 and 49 to provide the maximum working stroke.

A further advantage of the illustrative arrangement is that the twopantographs 40 and 40a are mounted on opposite sides of the spaced frameplates 46 and 47, thereby providing a relative wide spread between thebearings of the pantograph links, while still providing a relativelycompact overall assembly. This spacing of the two pantographs, combinedwith the relatively wide spread of the interconnections between thevarious pantograph links, provides desirable stability and rigidity tothe overall system, while still maintaining a relatively compact size.

For the purpose of further stabilizing the pantograph assembly 40, afirst stabilizing rod 50 is mounted in parallel with the-transfer arms30. The lower end of the stabilizing rod 50 is pivotally connected tothe gripper mounting plate 29, while the upper end of the rod ispivotally connected to an L-shaped bracket 51 which is rigidly mountedon a pin 52 interconnecting links 43 and 44 of the pantograph. As can beseen most clearly from the sequential views in FIGS. 5 and 6, thebracket 51 is always maintained in a fixed vertical-horizontalorientation, regardless of the position of the pantograph assembly, sothat the horizontal leg 53 of the bracket effectively maintains thegripper mounting plate 29 in a corresponding horizontal position. Thatis, the horizontal arm 53 of the bracket and the gripper mounting plate29 always remain horizontal, with the transfer arm 30 and thestabilizing rod 50 pivoting therebetween as'the pantograph assembly isdisplaced,

e.g., from the position illustrated in FIG. 5 to the positionillustrated in FIG. 6.

To further stabilize the pantograph assembly, and to maintain thebracket 51 in the desired orientation, the I vertical leg 54 of thebracket is connected to a second stabilizing rod 55, the upper end ofwhich is pivotally connected to a fixed point on the frame plate 46,with the lower end of the rod 55 being pivotally connected to the upperend of the vertical leg 54 of the bracket 51.

It will be appreciated that a similar arrangement of stabilizing rods isassociated with the second pantograph 4011, which is a complete mirrorimage of the first pantograph 40.

Turning next to the biaxial drive system that is mounted between theframe plates 46 and 47, for the purpose of driving the pins 48 and 49both horizontally and vertically, the drive system basically comprises apair of ball screw units operated by a pair of servo motors 62 and 72.The horizontal ball screw unit comprises a ball screw nut 60 threaded ona horizontal drive screw 61. The screw 61 is rotated in either directionby means of a reversible horizontal servo motor 62 which is rigidlymounted on the end of a housing having an upper section 63 and a lowersection 64. The housing formed by the sections 63 and 64 is mountedbetween the frame plates 46 and 47 for vertical movement relativethereto, as will be described in more detail below. When the horizontalservo motor is actuated to turn the screw 61, the ball screw nut 60 ismoved horizontally along the screw 61, with the direction of movementbeing determined by the direction of rotation of the drive screw 61 bythe reversible servo motor 62. The pins 48 and 49 which couple the ballscrew drive unit to the pantograph assembly are formed as integral partsof the ball screw nut 60, so that any movement of the nut 60 causes acorresponding movement of the pantographs 40 and 40a.

For the purpose of moving the pantograph in the vertical direction, asecond ball screw unit includes a ball screw nut 70 fixed to the upperhousing section 63 and threaded on a vertical drive screw 71. Rotationof the vertical drive screw 71 is controlled by means of a reversiblevertical servo motor 72 rigidly mounted on the frame plates 46 and 47via plate 72a. As the drive screw 71 is rotated by the motor 72, theball screw nut 70 is moved vertically along the screw 71, therebyeffecting vertical displacement of the entire horizontal ball screw unitincluding the housing sections 63 and 64 and the motor 62. Of course,the vertical movement of the horizontal ball screw unit also moves thepins 48 and 49 connected to the pantographs 40 and 40a, therebyeffecting corresponding vertical movement of the pantographs.

To permit vertical movement of the horizontal ball screw unit, the twohousing sections 63 and 64 are provided with a plurality of verticalguide rods 65, 66 and 67, 68 which are slidably mounted in correspondingbushings 65a, 66a, 67a, 68a within a plurality of corresponding guidecylinders 69, 70 and 71, 72. The guide cylinders 69-72 are rigidlymounted in a housing assembly including a pair of plates 73 and 74connected between the frame plates 46 and 47, substantially enclosingthe horizontal ball screw unit.

As can be seen in FIG. 3, windows are formed in opposite sides of thehousing formed by sections 63 and 64 to permit horizontal movement ofthe coupling pins 48 and 49, and a similar window, larger in thevertical dimension, is formed in the frame plates 46 and 47 to permitvertical movement of the coupling pins 48 and 49. Whenever the verticalservo motor 72 is operated, the entire horizontal ball screw unit,including the housing sections 63 and 64, is displaced vertically, withthe guide rods 65-68 sliding up and down within the bushings 65a-68a inthe guide cylinders 6972. The electrical system for controlling anddriving the servo motors 62 and 72 is synchronized with the main pressdrive system, a portion of which is illustrated in FIG. 9. Knowledge ofonly the main elements of a power press will permit full understandingof the present invention, and for the details of a commercial orpractical press, cross-reference may be made to the trade literature anddescriptive brochures of press manufacturers. Referring to H6. 10, thepress driving motor 80 is shown feeding astep down drive connectionincluding a belt 81 and pulleys 82, 83. Pulley 83 is connected to aclutch 85 having an input shaft 86 and an output shaft 87. The clutchoutput shaft 87 is coupled to an output pinion 90 which drives a pair ofintermediate gears 91, 92 which mesh with the main press drive gears 93,94

respectively. In a conventional press, such gears are connected to thepress pitman, and the movement and phasing of the slide are under thejoint control of timing cams and operating pushbuttons which, throughappropriate and interlocked circuitry, control the energization of theclutch 85.

One of the main press drive gears, in this case the gear 94, is meshedwith a power take-ofi' pinion 95 secured to a horizontally extendingpower take-off shaft 96. For driving the first transfer mechanism, thepower shaft is coupled to a servo cam box 97 to be described in moredetail below. The shaft 96 also drives a bevel gear 960, which mesheswith a bevel gear 98 having a horizontal shaft 99 which is coupled, viaa pair of bevel gears 100, 101, to a rotary cam limit switch drive shaft102. At is lower end the latter carries a worm gear 103 meshing with aworm wheel 104 which drives a timing cam assembly 105. The assembly 105maycorrespond to timing cams used for interlock purposes in a regularpower press, and is driven via bevel gears 106, 107 which rotate avertical shaft 108.

Returning now to the servo cam box 97, one of the cams in this box,illustrated as cam 110 in FIG. 9, drives a tachometer 111 (e.g., a modelNo. SU-780D-1 Servo-Tek tachometer) and a resolver 112, for the purposeof generating electrical input signals for controlling the servo motor62. More particularly, the tachometer 111 responds to rotation of thecam 110 to generate a d-c. signal representing both the magnitude anddirection of the velocity desired at the output of the servo motor 62.This d-c. signal from the tachometer 111 is supplied to an amplifier 113which drives the servo motor 62, with the magnitude of the d-c. signalbeing proportional to the magnitude of the velocity of the servo motor62, and the polarity of the d-c. signal determining the direction of thevelocity. More specifically, the slope of the cam 110 controls themagnitude and polarity of the d-c. output from the tachometer 1 1 1which causes the amplifier 113 to drive the servo motor 62.

In order to provide a position feedback loop to compensate for systemerrors, such as might be caused by drift of the amplifier 113 forexample, the resolver 112 is connected to a second resolver 114 drivenby the servo motor output screw 61. (It will be understood that bothresolvers 112 and 114 are electromechanical transducers which developoutput voltages proportional to the input voltages and sine of theangles of displacement of the input shaft.) As long as the mechanicaloutput displacement sensed by the resolver 1 14 corresponds to themechanical displacement of the cam 110, the two resolvers 112 and 114are balanced and, consequently, there is no output signal from theresolver 114. However, whenever a differential exists between the twomechanical inputs to the resolvers 112 and 114, the resolver 114generates an a-c. output signal which is supplied to a discriminator l15. The discriminator 115 converts the a-c. signal to a d-c. signal,which is'supplied to the amplifier 113 to compensate for any systemerrors.

For the purpose of preventing the servo-motor 62 from overshooting thecommanded displacement, which in turn would result in hunting of thesystem, the servo motor output screw 61 is mechanically coupled to atachometer 116 which produces a feedback signal to the servo motor 62.

In FIG. I1, one illustrative arrangement for providing a mechanicalconnection between the cam 110 in the servo cam box 97 and thetachometer MI and the resolver N2 is shown in more detail. In thisarrangement, the cam 11f) cooperates with a cam follower on the end of arack 121 which is biased against the cam surface by means of a spring122. As the rack I21 is moved linearly in response to rotation of thecam 110, the rack turns a pinion 123 which is connected at opposite endsto the mechanical input shafts of the tachometer 111 and the resolver112, respectively. As described previously, rotation of the input shaftsto the tachometer and resolver results in electrical output signalsrepresenting both the magnitude and direction of movement of the cam II0.

It will be understood that the control system of FIG. 9 controls onlythe servo motor 62, and that a similar control system, driven by asecond cam in the servo cam box 97, controls the vertical servo motor72. It will also be understood that the entire control system, includingboth the pantograph and the servo motors associated therewith, have beendescribed with reference to the control of only one transfer assembly,since the control systems for the other transfer assemblies areidentical, and thus the description of the one exemplary control systemis equally applicable to the control system as applied to any of theother assemblies.

Since the cam unit-97 controls the cyclic movement of the servo motors62 and 72, and thus the transfer assembly, it will be appreciated thatthe transfer assembly may be programmed to follow different cyclic pathssimply by substituting different cams in the unit 97. This is arelatively low cost substitution, in comparison with the overall system,and thus it does not impose any great economic burden on the user tohave different cams for each different part that is to be operated on bythe press line. Moreover, substituting one set of cams for another is arelatively quick and easy operation.

We claim as our invention:

1. A transfer system for an automatic press line comprising thecombination of a transfer arm having gripper means mounted on one endthereof for gripping the workpieces to be transferred, said transfer armbeing mounted on a pantograph comprising four pivotally interconnectedlinks for both vertical and horizontal movement, a pair of servo motorsoperatively connnected to said pantograph for effecting both verticaland horizontal movement of said pantograph, said pantograph and saidtransfer arm being dimensioned to amplify any movement of saidpantograph effected by said servo motors, and a servo control system foreach of said servo motors including program means connected to theautomatic press line for generating electrical control signals accordingto a predetermined program in synchronization with said press line, saidcontrol signals including both a velocity signal and a displacementsignal, means for driving said servo motor in response to said controlsignals, velocity feedback means responsive to the velocity of themechanical output of said servo motor for generating a velocity feedbacksignal, displacement feedback means responsive to the displacement ofthe mechanical output of said servo motor for generating a displacementfeedback signal, and means for combining said feedback signals with saidcontrol signals to modify the effect of said sigznals on said drivingmeans.

. A transfer system as set forth in claim 1 wherein said servo motorsare located at said transfer arm and said program means is located atthe drive means for the press line with said servo motors and saidprogram means being electrically connected.

3. A transfer system as set forth in claim ll wherein said program meansincludes a pair of cams connected to said press drive means means, oneof said cams varying the electrical signals controlling operation of oneof said servo motors, and the other cam varying the electrical signalscontrolling operation of the other servo motor.

4. A transfer system as set forth in claim 3 which includes a pair ofracks driven by said cams and a pair of pinions driven by said racks,and a pair of tachometers operatively connected to said pinions forgenerating electrical control signals in response to rotation of saidpinions.

5. A transfer system as set forth in claim 1 which includes a pair ofball screw drive units both connected to said pantograph at a commonconnecting point, said ball screw drive units being driven by said servomotors for pivoting said pantograph in response to operation of saidsen/o motors.

1. A transfer system for an automatic press line comprising the combination of a transfer arm having gripper means mounted on one end thereof for gripping the workpieces to be transferred, said transfer arm being mounted on a pantograph comprising four pivotally interconnected links for both vertical and horizontal movement, a pair of servo motors operatively connnected to said pantograph for effecting both vertical and horizontal movement of said pantograph, said pantograph and said transfer arm being dimensioned to amplify any movement of said pantograph effected by said servo motors, and a servo control system for each Of said servo motors including program means connected to the automatic press line for generating electrical control signals according to a predetermined program in synchronization with said press line, said control signals including both a velocity signal and a displacement signal, means for driving said servo motor in response to said control signals, velocity feedback means responsive to the velocity of the mechanical output of said servo motor for generating a velocity feedback signal, displacement feedback means responsive to the displacement of the mechanical output of said servo motor for generating a displacement feedback signal, and means for combining said feedback signals with said control signals to modify the effect of said signals on said driving means.
 2. A transfer system as set forth in claim 1 wherein said servo motors are located at said transfer arm and said program means is located at the drive means for the press line with said servo motors and said program means being electrically connected.
 3. A transfer system as set forth in claim 1 wherein said program means includes a pair of cams connected to said press drive means means, one of said cams varying the electrical signals controlling operation of one of said servo motors, and the other cam varying the electrical signals controlling operation of the other servo motor.
 4. A transfer system as set forth in claim 3 which includes a pair of racks driven by said cams and a pair of pinions driven by said racks, and a pair of tachometers operatively connected to said pinions for generating electrical control signals in response to rotation of said pinions.
 5. A transfer system as set forth in claim 1 which includes a pair of ball screw drive units both connected to said pantograph at a common connecting point, said ball screw drive units being driven by said servo motors for pivoting said pantograph in response to operation of said servo motors. 